feat(P1+P2): server-authoritative favorites/recent + pre-computed display fields

P1-A: Enrich FavoriteItemDto & RecentItemDto with item_name, item_size,
      item_mime_type, parent_id, modified_at via SQL LEFT JOINs — eliminates
      N+1 per-item fetches.

P1-B: Rewrite favorites.js as server-authoritative (724→380 lines).
      In-memory cache backed by GET /api/favorites; no localStorage.

P1-C: Rewrite recent.js as server-authoritative (341→249 lines).
      GET /api/recent + POST /api/recent/{type}/{id}; no localStorage.

P1 cleanup: Remove dead localStorage cleanup from auth.js logout,
            fix async clearRecentFiles in app.js.

P2: Add icon_class, icon_special_class, category, size_formatted to
    FileDto, FolderDto, FavoriteItemDto, RecentItemDto. New shared
    display_helpers.rs module centralises mime→icon/category/size logic.
    Frontend (ui.js, fileRenderer.js, favorites.js, recent.js) now reads
    pre-computed fields from the API with fallback defaults — eliminates
    5 duplicated mime→icon mapping blocks (~89 lines removed).

Also fixes: synthetic FolderDto in webdav_handler.rs, pre-existing
            missing item_name field in share_service.rs test.

Net: -307 lines across 15 files. cargo check: 0 errors, 0 warnings.
     cargo test display_helpers: 3/3 pass.

src/application/dtos/file_dto.rs

@@ -1,6 +1,8 @@
 use crate::domain::entities::file::File;
 use serde::{Deserialize, Serialize};
 
+use super::display_helpers::{format_file_size, mime_to_category, mime_to_icon_class, mime_to_icon_special_class};
+
 /// DTO for file responses
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct FileDto {
@@ -27,19 +29,40 @@ pub struct FileDto {
 
     /// Last modification timestamp
     pub modified_at: u64,
+
+    // ── Pre-computed display fields ──
+
+    /// FontAwesome icon CSS class (e.g. "fas fa-file-image")
+    pub icon_class: String,
+
+    /// Extra CSS class for icon styling (e.g. "image-icon", "" when default)
+    pub icon_special_class: String,
+
+    /// Human-readable file category (e.g. "Image", "Document")
+    pub category: String,
+
+    /// Human-readable formatted size (e.g. "3.27 MB")
+    pub size_formatted: String,
 }
 
 impl From<File> for FileDto {
     fn from(file: File) -> Self {
+        let mime = file.mime_type();
+        let size = file.size();
+
         Self {
             id: file.id().to_string(),
             name: file.name().to_string(),
             path: file.path_string().to_string(),
-            size: file.size(),
-            mime_type: file.mime_type().to_string(),
+            size,
+            mime_type: mime.to_string(),
             folder_id: file.folder_id().map(String::from),
             created_at: file.created_at(),
             modified_at: file.modified_at(),
+            icon_class: mime_to_icon_class(mime).to_string(),
+            icon_special_class: mime_to_icon_special_class(mime).to_string(),
+            category: mime_to_category(mime).to_string(),
+            size_formatted: format_file_size(size),
         }
     }
 }
@@ -47,9 +70,8 @@ impl From<File> for FileDto {
 // To convert from FileDto to File for batch handlers
 impl From<FileDto> for File {
     fn from(dto: FileDto) -> Self {
-        // Use constructor to create an entity from DTO
-        // Note: this should be simplified if File has a proper constructor
-        // If not, make the conversion as best as possible
+        // Display fields (icon_class, icon_special_class, category, size_formatted)
+        // are not part of the domain entity and are ignored.
         File::from_dto(
             dto.id,
             dto.name,
@@ -75,6 +97,10 @@ impl FileDto {
             folder_id: None,
             created_at: 0,
             modified_at: 0,
+            icon_class: "fas fa-file".to_string(),
+            icon_special_class: String::new(),
+            category: "Document".to_string(),
+            size_formatted: "0 Bytes".to_string(),
         }
     }
 }